Unless he is buying from another barrel maker its not possible to order small quantities of GB quality steel.
This is why so many small ML barrel makers use mill run steel.
Mill run, leaded 4140 (41L40) is little if any better than 12L14 for example.
Making certified high quality steel is far different than making mill run steel. High quality steel cannot be ordered in small lots since its made in furnace melt lots and sold that way. Small makers have to pool orders to get enough tonnage to make up a melt.
This from conversations with barrel makers and from my time a Shiloh.

Dan